Glass manufacturing, generally speaking, is a high temperature energy intensive operation where approximately sixty-five to seventy percent of the total energy used is consumed in the melting process. Typical gas or oil fired glass melting furnaces have thirty percent or more of their total input energy exhausted through the exhaust stack.
The fuel fired furnaces presently used for glass melting are either recuperative, regenerative, or direct fired. The recuperative type are typically smaller specialty glass furnaces; however, the regenerative types are basically the larger furnaces in which the regenerators take the form of brick work through which the combustion air is passed on its way to the area where the burners feed the fuel together with the combustion air into the furnace and through which the exhaust from the furnace passes on its way to the stack. The exhaust gases transfer their heat to the regenerator, or "checkers" in the form of bricks, as they pass through it. On the reverse cycle, the combustion air, which is clean air, brought in at ambient temperature, is passed through the regenerator on the other side; and thus picks up heat from the bricks, and in this way, preheats the air prior to its entry through the ports where the fuel is also introduced to cause combustion and melting of the glass in the furnace.
The more efficient regenerative furnace designs have combustion air preheating to a temperature of around 2300.degree. F. Nearly all glass furnaces used to melt container glass, which is soda-lime-silica glass, are of the regenerative type and have production capacities somewhere between 180 and 400 tons melted per day. As explained, in present day furnaces, the heat energy used and recycled in regenerative furnaces is through the alternate passing of the exhaust and combustion air through the regenerators which serve as heat storage devices.
A typical well designed furnace melting 183 tons per day, through experience, would require an energy input into the melter of 4 MM BTU/ton of glass melted in fuel and 2.2 MM BTU/ton in preheated air. The energy outputs are 2 MM BTU/ton in molten glass, 0.8 MM BTU/ton in melter radiation losses, and 3.4 MM BTU/ton out the exhaust ports. Of the 3.4 MM BTU/ton in the melter exhaust, 65% or 2.2 MM BTU/ton is recycled back into the melter as preheated air by the regenerator heat storage. 0.2 MM BTU/ton is lost as regenerator wall losses and 1 MM BTU/ton goes up the stack with the combustion products. Less efficient furnaces will have greater stack losses and accordingly greater fuel requirements. Waste heat recovery programs have been aimed at the approximately 1 MM BTU/ton melted which goes up the stack. Some of the familiar programs utilizing the waste heat stream for energy input are batch preheating, waste heat boilers, and organic Rankine cycle heat recovery systems. Batch preheating recycles more of the exhaust heat back into the melter with further melter fuel reduction. The other two programs are aimed at using the exhaust heat for some other beneficial use, such as steam for inplant use or for the production of electric power.
The regenerator exhaust heat from a typical regenerative furnace is present at a maximum temperature of about 950.degree. F. Since the minimum allowable stack exhaust temperature is about 450.degree. F., due to the condensation and corrosion problems if the exhaust gas temperature is less than this, the best the heat recovery system can do is collect about half the heat in the regenerator exhaust. When the collected heat is run through a Rankine cycle to produce power, at best 20% of this half is converted to work and the remaining 80% is rejected into the environment as low quality heat. The addition of the Rankine cycle to the above described well designed glass furnace would result in an output of about 29 KWH per ton melted. Whether the working fluid selected is steam or an organic compound, in the Rankine cycle roughly 80% of the collected heat must be thrown away in order to convert the remaining 20% into useful work.
An alternative approach for converting the regenerator exhaust heat into power is through the use of an externally fired Brayton cycle. The major difference between a Rankine cycle and a Brayton cycle is that the Brayton cycle uses a gaseous working fluid without condensation of the rejected heat stream.
Considering a typical regenerative furnace and the heat values that are present through the various portions of the furnace, reference can be made to FIG. 1, which is a schematic representation of a typical side port furnace. The arrows shown thereon indicate the direction of flow of air and gases, and considering that air at approximately 2200.degree. F. is entering at the upper left hand port and at the same time fuel is being introduced at this port as well, you will have an input into the preheated combustion air at 2200.degree. F. which will have a heat content value of 2.5 MM BTU/ton of glass melted and the fuel heat content equal to 4.5 MM BTU/ton melted. Thus, there is a total heat input quantity of 7 MM BTU/ton of glass melted. From this point on, heat quantities will be understood to be normalized on a per ton of glass melted basis.
There is in a typical furnace a heat loss through the furnace wall of about 1 MM BTU. In addition to the exhaust, the glass that is issuing from the furnace will carry away heat quantities of about 2 MM BTU, thus leaving the heat quantity which is exhausted as combustion exhaust from the melter at 4 MM BTU. This heat will raise the temperature of the upper end of the checker works (on the right side in FIG. 1) to approximately 2600.degree. F. The hot exhaust gas passes down through the right hand checkers and enters the lower canal at approximately a temperature in the range of 900.degree.-1000.degree. F. with a heat content equal to about 1.5 MM BTU. This exhaust gas then exits the canal and enters the stack at a temperature in the range 550.degree. to 850.degree. F. The combustion air is brought into the lower end of the left hand regenerator or checker works and enters with a heat content of 0 since it is atmospheric air at approximately 60.degree. F., the enthalpy reference condition. This air is then heated to 2200.degree. F. during its upward flow. It should be remembered that this cycle reverses itself, in that the combustion is reversed, and occurs at the other side with a reversal of the valve connections of the incoming air and the exiting exhaust to the stack. This is illustrated in FIG. 1.
In FIG. 2, there is shown this same typical side port furnace, but with one additional element, and that is the pre-preheating of the incoming air to the regenerator which is used to preheat the combustion air. Thus it can be seen that inlet air, rather than at 60.degree. F. as explained with respect to FIG. 1, is now shown as being at 750.degree.-800.degree. F. This has a heat content of 0.6 MM BTU. This added preheat, then, will increase the heat content of the combustion air to 2.6 MM BTU so that after passing through the left hand checkers the temperature of the combustion air will be generally 2300.degree. F. To this then could be added a fuel heat value of 4.4 MM BTU. It should be noted that this is 0.1 MM BTU less than that required with respect to FIG. 1. Again, the heat loss value through the melter wall would be 1 MM BTU and the glass issuing from the furnace would carry away heat quantities of about 2 MM BTU leaving an exhaust heat content of 4 MM BTU as it did in the earlier example with respect to FIG. 1. However, it should be kept in mind that since 750.degree.-800.degree. F. air is being introduced to the lower end of the checkers, the lower portions of the checkers will store more heat and be at a considerably greater temperature than without the 750.degree.-800.degree. F. air inlet. Thus, the temperature of the exhaust which arrives at the bottom of the right hand checker will be in the range of 1250.degree.-1350.degree. F. with a heat content of 2 MM BTU. This is then an increase in heat value of approximately 0.5 MM BTU; however, since the lower end of the checkers is connected to the stack, it can be seen that the exhaust entering the stack will be at a temperature of 800.degree.-1200.degree. F. It is to be noted that the preceding and following temperature and heat content relationships are a clear function of furnace system design and operation.
Taking the same typical furnace as shown in FIG. 2 and adding to it a Brayton cycle heat recovery system, it can be seen that a considerable saving may be effected. An elevated pressure heat exchanger is illustrated in FIG. 3 as being used in combination with the Brayton cycle heat recovery system wherein ambient air is brought into a compressor which in turn is driven from a turbine coupled thereto with the turbine being powered by the expansion of the compressed air from the heat exchanger with the exhaust from the turbine providing the 750.degree.-800.degree. F. preheat combustion air for the operation of the main melter. Also, in this situation the turbine will not only drive the compressor but may also drive an electric generator which will generate a certain amount of electric power.
As shown in FIG. 3, the Brayton cycle air turbine T, which is illustrated, will receive heated air from the heat exchanger at approximately 1300.degree. F., with an expansion taking place in the turbine to drive the compressor, with the exhaust air from the turbine at &gt;750.degree. F., being fed into the melting system as the pre-preheat combustion air. This cycle, as compared to FIG. 2, serves to provide preheat air which is in excess of that required for combustion and reduces heat content of the exhaust to the stack while still maintaining the temperature in the acceptable range of 450.degree.-500.degree. F. The other temperature figures are essentially the same as the typical regenerative furnace with preheat air as shown in FIG. 2. The electric power generated by the Brayton cycle will thus be nearly double that by a Rankine cycle on a similar furnace.
The cogeneration of electricity from glass furnace waste heat is not a new concept per se and was a topic at the 9th Energy Technology Conference presented in February 1982 and is the subject of articles presented at such conference. One such article is reproduced in the minutes of the conference at pages 375-388, authored by J. G. Hnat, J. S. Patten and J. C. Cutting. This article explains the relationship of waste heat recovery systems using a steam Rankine cycle with the heat recovery medium being pressured steam, an organic Rankine cycle heat recovery system using toluene as the working fluid, and as a third system, an indirectly heated, positive pressure Brayton cycle heat recovery system. It is this type of system to which applicant's invention is directed, and which applicant will describe in detail hereinafter.
In addition to the article by Hnat et al, a second article by James G. Hnat, J. S. Patten, and Praven R. Sheth, all of Industrial Energy Research Division of Gilbert/Commonwealth of Reading, Pa., was presented at the 1981 Industrial Energy Conservation Technology Conference and Exhibition in Houston, Tex. Apr. 26-29, 1981. This article describes Rankine and Brayton cycle cogeneration from glass melting. Here again, the systems which were evaluated and studied were a conventional steam Rankine cycle, an organic Rankine cycle, an indirectly heated pressurized Brayton cycle, and a sub-atmospheric Brayton cycle. The indirectly heated pressurized Brayton cycle is one which is most pertinent to the present invention. The study outlined in this article dealt with the positive pressure Brayton cycle, with the flue gases from the furnace transferring heat to compressed air delivered by a compressor at 38.7 psi and 265.degree. F. The heated air was then expanded through a single stage turbine which drove both a compressor and a generator. The exhaust air from the turbine was delivered to the regenerator as preheated combustion air. A turbine expansion ratio of 2.5 to 1 was used for the positive pressure Brayton cycle based upon a review of data published on heat recovery turbo expanders used in fluid catalytic cracking processes. Studies on waste heat recovery by Garrett Airesearch Manufacturing Company as cited in this article by Hnat, Patten and Sheth suggest using an expansion ratio of 3.5 to 1. The turbine and compressor efficiency assumed by Hnat et al were 85% and 87%, respectively. Heat exchange parameters in the range of 70% to 92.5% were considered, and the impact on cycle performance examined. The conclusions reached by the authors about performance and cost comparisons of this study indicate that the Brayton cycle generates progressively less electric power as heat exchanger effectiveness decreases. The authors acknowledged conclusions by others, viz., Rose & Colosimo, Power, Energy Systems Guide Book, August 1980, pages 42-43, that the minimum turbine inlet temperature for effective Brayton cycle performance was on the order of 1300.degree. F. Therefore, the authors were not surprised that the electrical power conversion efficiencies predicted were low. The authors raised assumed heat exchanger effectiveness from 70% to 85% and calculated a significant increase in power for the Brayton cycle. However, the power output was still observed to be substantially less economically attractive than Rankine cycle systems. It should be remembered that all of these are assumptions based on factors which do not necessarily represent the true conditions which would be obtained in a plant. As is the subject of this invention, the authors did not recognize the real potential of the positive pressure Brayton cycle in furnace waste heat recovery.
Another article published in 1979 by the American Chemical Society is the Indirect Brayton Energy Recovery System authored by B. E. Lampinen, R. R. Gutowski, A. Topouzian and M. A. Pulick of Ford Motor Company, Dearborn, Mich. This article describes the simple Brayton Cycle when applied to exhaust gas at 1300.degree. F. with exhaust to ambient at 410.degree. F. The Brayton cycle takes air at 100.degree. F. into a compressor, passes the exit air from the compressor to the heat exchanger and from the heat exchanger to the turbine with a preheat air exiting from the turbine at 930.degree. F. This article also describes an IBERS cycle, which is a variation of the simple Brayton cycle. In the IBER system, hot exhaust gases are expanded from atmospheric to sub-atmospheric pressure directly through a turbine. Exhaust gas leaving the turbine is then passed through a heat exchanger where it is cooled to 200.degree.-300.degree. F. The cooled gas is then compressed back up to atmospheric pressure by the compressor coupled to the turbine. In this study it appears that the IBERS cycle is considered to have advantages over the simple Brayton cycle for recovery of waste heat. This IBERS cycle, or Indirect Brayton Energy Recovery System, is one in which the exhaust gas from the furnace itself is passed through the turbine and then the output from the turbine is passed through the primary of the heat exchanger then back to the compressor where the exhaust temperature is 460.degree. F. By and large the remainder of the article deals with the advantages of the IBERS cycle over the simple Brayton cycle. The IBERS cycle has 2 major disadvantages for use in waste heat recovery systems. These are, first, that one is forced to run the dirty furnace exhaust gas through the turbo machinery or alternatively develop a hot clean up method, and, second, that the low pressure operation requires a larger physical size in turbo machinery and heat exchange equipment.